From the Guidelines
The recommended Thyroid-Stimulating Hormone (TSH) level when trying to conceive should be below 2.5 mIU/L. For women with known thyroid disorders who are planning pregnancy, maintaining TSH levels between 0.5-2.5 mIU/L is optimal for fertility and healthy pregnancy outcomes, as suggested by some investigators 1. If you have elevated TSH levels, your doctor may prescribe levothyroxine to lower TSH to the target range, as the potential benefit-risk ratio of levothyroxine therapy justifies its use 1. Dosage is individualized based on your specific levels, typically starting at 25-50 mcg daily and adjusted every 4-6 weeks until optimal levels are achieved.
Importance of Thyroid Function Testing
It's essential to have your thyroid function tested before conception if you have a history of thyroid issues, family history of thyroid disease, or symptoms of thyroid dysfunction. This recommendation exists because even mild hypothyroidism (elevated TSH) can interfere with ovulation, implantation, and early embryonic development. Additionally, proper thyroid function during early pregnancy is crucial for fetal brain development, as the baby relies on maternal thyroid hormones during the first trimester before developing its own thyroid function.
Key Considerations
- The upper limit of normal for serum TSH concentration is suggested to be 2.5 mIU/L in a population rigorously screened to exclude thyroid disease or drugs that influence thyroid function 1.
- Serum TSH concentrations between 2.5 and 4.5 mIU/L may be due to minor technical problems in the TSH assay, circulating abnormal TSH isoforms, or heterophilic antibodies; normal individuals with serum TSH concentrations in this range would be misidentified as having hypothyroidism 1.
- Pregnant women or women of childbearing potential planning to become pregnant who are found to have elevated serum TSH should be treated with levothyroxine to restore the serum TSH concentration to the reference range 1.
From the Research
Thyroid-Stimulating Hormone (TSH) Levels When Trying to Conceive
The recommended TSH level when trying to conceive is a topic of ongoing research and debate. Several studies have investigated the optimal TSH range for infertile women attempting conception.
- A study published in 2024 2 found that women with unexplained infertility and high-normal TSH levels (2.5-5 mIU/L) who were treated with levothyroxine had a higher conception rate, but a lower live birth rate compared to untreated women.
- Another study published in 2015 3 found that there was no difference in implantation, live birth, or miscarriage rates among women with TSH levels ≤2.5 mIU/L undergoing in vitro fertilization (IVF).
- A study published in 2018 4 found that women with unexplained infertility had higher TSH levels within the normal range compared to women with a normal fertility evaluation, suggesting that high-normal TSH levels may be associated with unexplained infertility.
- Earlier studies, such as one published in 1987 5, focused on the assessment of thyroxine dosage in patients with primary hypothyroidism and found that serum TSH concentrations were not very useful for this purpose.
- A more recent study published in 2025 6 investigated the levels of reverse T3 (rT3) in patients with hypothyroidism on different thyroid hormone replacement therapies, but did not directly address the optimal TSH range for conception.
Optimal TSH Range for Conception
Based on the available evidence, the optimal TSH range for conception is not clearly established. However, some studies suggest that:
- TSH levels ≤2.5 mIU/L may be beneficial for women undergoing IVF 3.
- High-normal TSH levels (2.5-5 mIU/L) may be associated with unexplained infertility, and treatment with levothyroxine may improve conception rates, but may not necessarily improve live birth rates 2.
- Further research is needed to determine the optimal TSH range for conception and to investigate the effects of treating high-normal TSH levels on reproductive outcomes.